* frontend, distributor: Add tenant ID validation
Use the same validation than in Mimir for consistency.
In addition, reject empty IDs ("").
* Validate tenantID everywhere except in frontend handlers
We can't do it in frontend handlers because we need to wait for the multitenant middleware,
but we rely on the validation middleware.
Validating everywhere follows the same pattern as Mimir and ensures we don't miss any edge case.
* Check status codes
* Add test for user overrides API
* add comment
